1. Field of the Invention
The present invention relates to a semiconductor laser element including an S-ARROW (Simplified Antiresonant Reflecting Optical Waveguide) structure, in which light in a fundamental transverse mode is confined within a width determined by a pair of elongated high-refractive-index layers separated from each other. The present invention also relates to a process for producing a semiconductor laser element including an S-ARROW structure.
2. Description of the Related Art
Currently, semiconductor laser elements are widely used as light sources for optical communication, optical disk devices, and the like, since laser light can be converged to a diffraction limit. However, only a portion of light emitted from each semiconductor laser element which is in phase at a light-emission end facet can be converged to a diffraction limit. The emission of light in phase at a light-emission end facet of a semiconductor laser element is called emission in a fundamental transverse mode. On the other hand, when light components having various phases coexist (i.e., light components oscillating in higher harmonic transverse modes are mixed) in light emitted from a semiconductor laser element, the light cannot be converged to a diffraction limit.
It is known that the operation in a fundamental transverse mode is more stable when an emission cross section is smaller since the mixture of higher harmonic transverse modes can be more effectively suppressed when the emission cross section is smaller. Therefore, the waveguides in the semiconductor laser elements which are required to emit light in a fundamental transverse mode are arranged to have a thickness of 1 micrometer or smaller and a width of about 2 to 4 micrometers. In particular, it is empirically known that the yield rates of the semiconductor laser elements which stably emit laser light in a fundamental transverse mode increase when the widths of the waveguides are reduced.
Nevertheless, when the emission cross section of a semiconductor laser element is reduced by reducing the width of the waveguide in the semiconductor laser element, the optical density at the light-emission end facet of the semiconductor laser element necessarily increases. The increase in the optical density causes deterioration of the constituent materials of the semiconductor laser element, and becomes a factor which decreases the lifetime of the semiconductor laser element.
In other words, the stabilization of the fundamental transverse mode (by reduction of the cross section of the waveguide) is in a trade-off relationship with the increase in the optical output since it is necessary to increase the cross section of the waveguide for increasing the optical output. Therefore, overcoming this problem is a significant challenge in the current research and development of semiconductor laser.
As an attempt to overcome the above problem, an S-ARROW structure is proposed, as disclosed by H. Yang et al. in xe2x80x9cHigh-power single-mode simplified antiresonant reflecting optical waveguide (S-ARROW),xe2x80x9d IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 10, No. 8, August 1998, pp. 1079-1081. Although this structure realizes emission in a fundamental transverse mode, the width of the emission cross section (the lateral width of the waveguide) can be increased to about 6 micrometers, which is 1.5 to 2 times greater than those in the conventional structures. Therefore, when the semiconductor laser element having an S-ARROW structure is used, it is possible to expect increase in the maximum optical output.
The reasons why the semiconductor laser elements having an S-ARROW structure can emit light in a fundamental transverse mode are explained below.
FIG. 10A shows a cross section (which is perpendicular to the laser propagation direction) of an essential portion of a semiconductor laser element having an S-ARROW structure. In the semiconductor laser element of FIG. 10A, a lower cladding layer 38 made of n-type InGaP, an SCH (separate confinement heterostructure) layer 37 being made of InGaAsP and including an InGaAs quantum-well active layer, upper cladding layers 36 and 32 made of p-type InGaP, an etching stop layer 35 made of n-type GaAs, a current stopping layer 34 made of n-type AlInP, guide portions 33 being made of n-type GaAs and having a thickness of, for example, 0.25 micrometers, and a contact layer 31 made of p-type GaAs are formed on an n-type GaAs substrate 39.
GaAs, which is used for forming the guide portions 33, has a relatively high refractive index compared with those of the constituent materials around the guide portions 33. Therefore, the equivalent refractive index in the direction parallel to the SCH layer 37 has a distribution as indicated in FIG. 10B. That is, the equivalent refractive index in the SCH layer 37 is high under the guide portions 33, and low in the other regions of the SCH layer 37.
In the above waveguide structure, the width A of each of the guide portions 33 is determined so that light in the fundamental transverse mode is confined in a region of the SCH layer 37 under the gap between the guide portions 33, and light in higher harmonic transverse modes is not confined in and leaks out from the region under the gap between the guide portions 33. In the Yang reference, the width A of each of the guide portions 33 is 0.85 micrometers, and the width B of the gap between the guide portions 33 is 6.5 micrometers.
Due to the provision of the current stopping layer 34, current which is supplied for generating laser is injected into only the region under the gap between the guide portions 33, and realizes laser gain in only the region under the gap between the guide portions 33.
Thus, only the light in the fundamental transverse mode is confined in the region under the gap between the guide portions 33, and it is possible to obtain a sufficient gain in the fundamental transverse mode. On the other hand, the light in higher harmonic transverse modes is not confined in the region under the gap between the guide portions 33. Therefore, it is impossible to obtain a substantial gain in higher harmonic transverse modes. As a result, the light in the fundamental transverse mode is dominantly emitted from the semiconductor laser element of FIG. 10A, and the semiconductor laser element of FIG. 11A can operate in a stable fundamental transverse mode even when the output power is high.
Nevertheless, when the S-ARROW structure is manufactured by the conventional technique, the yield rate is necessarily lowered for the following reasons. In order to explain the reasons, first, the conventional process for producing the semiconductor laser element including the S-ARROW structure is explained with reference to FIGS. 11 to 14.
First, as illustrated in FIG. 11, the n-type InGaP lower cladding layer 38, the SCH layer 37 being made of InGaAsP and including the InGaAs quantum-well active layer, the p-type InGaP upper cladding layer 36, the n-type GaAs etching stop layer 35, the n-type AlInP current stopping layer 34, and an n-type GaAs guide layer 33xe2x80x2 having a thickness of 0.25 micrometers are formed in this order on the n-type GaAs substrate 39 by organometallic vapor phase epitaxy.
Next, in a first photolithography step and an etching step, the outside portions of the n-type GaAs guide layer 33xe2x80x2 are removed so as to leave a portion of the n-type GaAs guide layer 33xe2x80x2 including the guide portions 33 in the semiconductor laser element of FIG. 10A. Thus, a layered structure having a cross section as illustrated in FIG. 12 is obtained.
Then, in a second photolithography step, as illustrated in FIG. 13, a resist pattern 40 is formed over the layered structure of FIG. 12 except for an area corresponding to the gap (having the width B) between the guide portions 33 in the semiconductor laser element of FIG. 10A.
Thereafter, the layered structure of FIG. 13 is etched from the upper side until the p-type InGaP upper cladding layer 36 is exposed. Thus, a groove M is formed in a position corresponding to the center of the waveguide, and a layered structure having a cross section as illustrated in FIG. 14 is obtained.
Subsequently, the resist pattern 40 is removed, and then the p-type InGaP upper cladding layer 32 and the p-type GaAs contact layer 31 are formed by crystal growth. Thus, the semiconductor laser element as illustrated in FIG. 10A is obtained.
However, when the width B of the gap between the guide portions 33 is different from a predetermined width, or the gap is formed in a position different from a predetermined position, due to variations in the widths of the respective guide portions 33 or in the positions of the guide portions 33, light in modes other than the fundamental transverse mode remains in the region of the SCH layer 37 under the gap between the guide portions 33. Therefore, the semiconductor laser element emits light in the modes other than the fundamental transverse mode, or light in the fundamental transverse mode does not satisfactorily propagate in the region of the SCH layer 37 under the gap between the guide portions 33, so that the oscillation threshold current in the fundamental transverse mode rises. Specifically, even when the positions or widths of the guide portions 33 deviate by 0.1 to 0.2 micrometers from predetermined positions or widths, the characteristics of the semiconductor laser element deteriorate.
When the S-ARROW structure is formed by the conventional process explained before, it is particularly necessary to form the resist pattern 40 with a very high precision of about 0,1 micrometers in the second photolithography step so that the groove M is formed in the predetermined position as illustrated in FIG. 14. That is, if the opening of the resist pattern 40 is not precisely located at the center of the guide portions 33, the gap between the guide portions 33 is formed in a position different from the predetermined position. In addition, if the width of the opening of the resist pattern 40 does not have a predetermined width, the width B of the gap between the guide portions 33 becomes different from the predetermined width. Therefore, it is necessary to precisely control the position of the opening of the resist pattern 40, relative to the position of the aforementioned portion of the n-type GaAs guide layer 33xe2x80x2 in the layered structure of FIG. 13. However, it is difficult to form the resist pattern 40 with high precision. Thus, according to the conventional technique, it is impossible to manufacture, at a high yield rate, the semiconductor laser element in which the gap between the guide portions 33 and the positions of the guide portions 33 are formed with high precision.
An object of the present invention is to provide a process by which a semiconductor laser element including an S-ARROW structure can be produced at a high yield rate.
Another object of the present invention is to provide a semiconductor laser element which includes an S-ARROW structure, and can be produced at a high yield rate.
(1) According to the first aspect of the present invention, there is provided a process for producing a semiconductor laser element. The process comprises the steps of: (a) forming on a substrate a plurality of layers including an active layer, a first cladding layer, and a current stopping layer in this order, where the first cladding layer has a first refractive index; (b) forming on the current stopping layer a mask having a pair of parallel openings extending in a laser propagation direction and having identical widths; (c) etching the current stopping layer by using the mask so as to form a pair of first parallel grooves extending in the laser propagation direction in the current stopping layer and a ridge between the pair of first parallel grooves; (d) etching off a pair of portions of the first cladding layer being located at the bottoms of the pair of first parallel grooves and having a predetermined depth so as to form a pair of second parallel grooves in the first cladding layer, and etching off a portion of the current stopping layer located in the ridge; (e) filling the pair of second parallel grooves with a material having a second refractive index higher than the first refractive index; and (f) forming a second cladding layer so as to cover the pair of second parallel grooves and a portion of the first cladding layer located between the pair of second parallel grooves.
Preferably, in the process according to the first aspect of the present invention, the plurality of layers may further include an etching stop layer located at the predetermined depth so that etching of the first cladding layer in step (d) stops at the etching stop layer.
The material having the second refractive index may be formed in only the pair of second parallel grooves. Alternatively, the material may be uniformly formed on the entire exposed area when the pair of second parallel grooves is filled with the material in step (e).
(2) According to the second aspect of the present invention, there is provided a semiconductor laser element comprising: a substrate; a plurality of layers being formed on the substrate and including an active layer, a first cladding layer, and a current stopping layer; and a second cladding layer. In the plurality of layers, the active layer, the first cladding layer, and the current stopping layer are formed in this order. The current stopping layer has an opening extending in a laser propagation direction and being located above a center portion of a waveguide, and a pair of parallel grooves extending in the laser propagation direction are formed at positions corresponding to side edges of the opening in the first cladding layer, and filled with a material having a refractive index higher than the refractive index of the first cladding layer. The second cladding layer is formed over the pair of parallel grooves and a portion of the first cladding layer located between the pair of parallel grooves.
Preferably, the semiconductor laser element according to the second aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii).
(i) The conductivity types of the current stopping layer and the first cladding layer may be opposite so that a current stopping structure is realized by a pn junction between the current stopping layer and the first cladding layer.
(ii) The material may be a semiconductor material.
(iii) The active layer may be made of an InGaAs material.
(3) The process according to the first aspect of the present invention and the semiconductor laser element according to the second aspect of the present invention have the following advantages.
In the process according to the first aspect of the present invention, the pair of first parallel grooves are formed in the current stopping layer by etching using the mask having the pair of parallel openings extending in the laser propagation direction and having identical widths. Then, a pair of second parallel grooves are formed in the first cladding layer by etching off the portions of the first cladding layer being located at the bottoms of the pair of first parallel grooves and having a predetermined depth, and a portion of the current stopping layer located in the ridge left between the pair of first parallel grooves is also etched off. That is, both of the pair of first parallel grooves and the pair of second parallel grooves are formed by etching.
Since the etching rate in each material is identical, the portions of the current stopping layer exposed through the pair of openings of the mask are etched at an identical etching rate, and therefore the widths and the depths of the pair of first parallel grooves formed in the current stopping layer become equal in a self-aligned manner. Similarly, the widths and the depths of the pair of second parallel grooves formed in the first cladding layer also become equal. Therefore, when the pair of openings in the mask are accurately formed on the current stopping layer, the widths and the depths of the material filling the pair of second parallel grooves become accurately identical in a self-aligned manner. Thus, it is possible to produce a semiconductor laser element having a high-precision S-ARROW structure which enables confinement of light in a fundamental transverse mode within a width determined by the pair of high-refractive-index layers (guide portions) being separated by a predetermined gap and extending in the laser propagation direction.
As explained above, by the process according to the first aspect of the present invention, it is possible to accurately form a pair of grooves having identical shapes in a self-aligned manner by etching when the pair of openings in the mask are accurately formed on the current stopping layer. Therefore, high precision position control is not required in formation of layers after the formation of the mask. Thus, it is possible to manufacture semiconductor laser elements having an S-ARROW structure at a high yield rate.
Further, when the plurality of layers further include an etching stop layer located at the predetermined depth so that etching of the first cladding layer in step (d) stops at the etching stop layer, it is possible to easily adjust the depth of the pair of parallel grooves in the first cladding layer.
The semiconductor laser element according to the second aspect of the present invention can be produced by the process according to the first aspect of the present invention. Therefore, the semiconductor laser element according to the second aspect of the present invention has an S-ARROW structure in which a pair of guide portions made of a high-refractive-index material are accurately formed. Thus, the semiconductor laser element according to the second aspect of the present invention can stably emit laser light in a fundamental transverse mode, and the oscillation threshold current in the fundamental transverse mode can be maintained in a low-current range.